Gelatin has long been known as an emulsion or dispersion base and carrier for dyes and other photographically active materials. For such applications it is important that the cast gelatin layers set as rapidly as possible without detriment to the forming film. In applications such as curtain-coating, a strong correlation exists between the rheology of gelatin and the critical speed for the onset of air entrainment. Highest coating speeds are obtained with coating fluids having the greatest shear thinning behavior. In those applications in which gelatin layers contain silver halide, even when high grade gelatins are employed, sedimentation of the silver halide often occurs before the gelatin layer sets because of the low viscosity of the casting solution.
Attempts to overcome these difficulties and increase the setting speed of gelatin by adding thickeners such as polystyrene sulphonic acid have been unsuccessful since thickeners frequently cause surface defects during casting. Alternatively, the use of hardenable materials to harden photographic gelatin layers chemically and thereby adjust the degree of swelling, increase the melting point, and increase mechanical strength has also been proposed. Unfortunately, hardeners introduce undesirable effects such as defective crystallization, adhesion difficulties, layer separation due to the varying lateral swelling of the individual layers, and other problems which render the material useless. Such problems are exacerbated when casting requirements make it necessary to employ such short drying times and/or such high drying temperatures that the ordered helical structures which develop during gelatin layer formation can only be partially formed.
Many attempts have been made to overcome such difficulties. For example, U.S. Pat. No. 4,421,847 describes a process for chain lengthening gelatin by partial hardening. However, the "partial hardening" referred to is a cross-linking reaction which results in the formation of insolubles and renders the material useless, particularly for photographic applications. Likewise, U.S. Pat. Nos. 3,642,486 and 3,539,644 describe the use of hardening agents to produce higher molecular weight gelatin. The resulting product is gelled and insoluble.
Some have proposed the addition of quickly hardenable materials such as chrome acetate or materials which actuate the hardening reaction of gelatin which otherwise takes place only slowly. For example, U.S. Pat. No. 2,652,345 suggests the addition of formaldehyde and treatment with gaseous ammonia. U.S. Pat. No. 2,996,405 proposes the addition of a mixed styrene-aminomaleic acid polymer and treatment with ammonia vapor. The reaction of gelatin with thiolactones to provide gelatin compositions stable in the acid range but quickly hardenable in alkaline media is described in U.S. Pat. No. 3,171,831. However, high pH values accelerate chemical cross-linking which is disadvantageous in photographic processes and renders the gelatin compositions useless in multilayer materials where crosslinking causes adhesion defects.
In another approach, the yield of higher viscosity, higher molecular weight gelatin obtained from lime processed ossein stock can be substantially increased by controlling the time and temperature of the alkaline extractions. However, significantly lower yields of gelatin and higher amounts of poorer quality gelatin are thus produced, resulting in low gel strength, higher color and lower clarity. Alternatively, the acid processing of ossein yields gelatins which have a much lower viscosity than gelatins obtained by lime processing. Gelatins derived from pigskin are normally obtained by the acid process and have the same low viscosities as acid processed ossein gelatins.
Accordingly, it is important to provide a gelatin having a faster setting time, higher viscosity, and higher molecular weight at the same gelatin concentration while retaining its solubility characteristics.